1. Field of the Invention
This invention relates to polymer compositions, and in a more specific, though non-limiting aspect, to additive materials which beneficially modify the properties exhibited by polymer compositions which undergo carbonization upon pyrolysis, such properties being, most notably, rate of consumption and quantity of smoke evolution.
2. Brief Description of the Prior Art
Much effort has been expended in discovering and developing useful fire retardant and smoke retardant polymer compositions, and numerous examples of these compositions have appeared in the literature, with some of them being in commercial use. As technology has become more sophisticated, various additives which affect the pyrolysis of polymers have been evaluated on the basis of an increasing number of objectives. Among these objectives are low cost of additives, low rate of polymer consumption, low quantity of smoke evolution, improved char formation characteristics, improved smoke compositions in the sense of reduced toxicity, and improved processibility. Since halogenated polymers produce dense smoke and some noxious hydrogen halides upon pyrolysis, much effort has recently been focused on reducing the quantity of smoke evolved in the pyrolysis of such polymers. Dense smoke seriously hampers fire fighting efforts, constitutes a real danger of asphyxiation or suffocation, and can obscure escape routes from a burning building or vehicle.
Various materials have heretofore been proposed as additives to polymers for the purpose of reducing the amount of smoke which is produced upon pyrolysis of these materials, and for reducing the rate at which the material is consumed. The production of smoke and the consumption rate are related, of course, in that the total volume of smoke produced over a given period of time will be a function of both the amount of smoke yielded per unit of material consumed during pyrolysis, and also the rate at which the material is consumed during pyrolysis.
Iron oxide has frequently been added to polyvinylchloride formulations, alone or in combination with other materials, to reduce the quantity of smoke evolved during pyrolysis of the polymer. The proposed types and amounts of iron oxide which have been said to be useful have varied widely. Thus, U.S. Pat. No. 4,055,538 discloses that red iron oxide or yellow iron oxide having an average particle size of from about 0.01 to about 800 microns can be utilized in combination with melamine molybdate as a smoke retardant additive to rigid PVC and polyvinylidine chloride polymer compositions. The rigid PVC can be filled or unfilled. The additive composition can contain as much as 99% by weight iron oxide, and from about 0.1 to about 20 phr of the additive composition is used in the polymer. Other patents disclosing the use of iron oxides for smoke reduction in polymers are U.S. Pat. Nos. 3,983,086; 3,821,151, and 3,993,607. Still other patents teach the use of iron oxide in combination with other metal oxides. For example, U.S. Pat. No. 3,903,028 discloses the use of iron oxide in combination with either copper oxide or molybdenum oxide in reducing smoke evolution from PVC during pyrolysis.
Iron oxides have thus been used in various forms and combinations as smoke suppressants in plastic. It has been recognized as highly probable that such metal oxides, when present in halogenated polymers, are converted to metal halides by hydrogen halide generated during pyrolysis of the polymer. Thus, it is speculated that iron oxide added to PVC is converted to iron chloride by hydrogen chloride liberated during the burning. It has also been implied that such metal halides catalyze smoke suppression. It is known, however, that iron chloride is an excellent dehydrohalogenation catalyst, and therefore causes premature degradation of halogenated polymers when used directly, instead of through conversion from iron oxide initially placed in the polymer.
U.S. Pat. No. 4,013,815 to Dorfman proposes to use up to 5 weight percent iron or iron compound in a halogen-containing polyester of a polycarboxylic compound and a polyhydric alcohol for smoke suppression purposes. A preferred form of iron compound for smoke suppression is a ferric sulfate hydrate, and iron oxide and iron carboxylates are also mentioned. Ferric chloride and ferric oxychloride are described as having desirable fire retardant properties, but adversely affecting the stability or cure of the polymer compositions. Such iron compounds are said to be useful in this context only in an inactive form, such as when encapsulated or complexed with another compound. Encapsulating the compounds with gelatin or other polymeric materials, or alternatively, complexing the ferric chloride or ferric oxychloride with nitro compounds or aromatic ethers or amines is specifically suggested. The example cited is an amine salt of ferric chloride.
Another Dorfman patent (U.S. Pat. No. 3,983,185) describes the use of a fire retardant metallic compound added to a halogen-containing polyester of a polycarboxylic acid and a polyhydric alcohol. The polyester contains more than 4 weight percent of halogen. The fire retardant additive is either an iron compound, an antimony compound and/or a copper compound. Iron salts said to be suitable include, inter alia, ferric sulfate hydrates, ferric fluoride, ferrous fluosilicate, ferrous hydroxide, ferrous sulfate, ferrous tartrate, ferrous stannate, ferrous chloride and ferrous ammonium sulfate. Iron oxides are described as especially useful compositions. Suitable copper salts include cupric chloride, cupric bromide, copper chloride dihydrate, copper sulfate pentahydrate, copper hydroxide chloride, basic copper chloride, copper oxychloride, copper carbonate and copper borate. Especially useful are the copper oxides. The antimony compounds described include, among others, antimony trioxide, antimony pentoxide, antimony sulfides, antimony tribromide, antimony trichloride, antimony tetrachloride, antimony trifluoride, antimony pentafluoride, antimony triiodide, antimony pentaiodide, antimony oxychloride and antimony trisulfate. This patent further indicates that while many of these compounds have fire retardant properties, they may adversely affect the stability or cure of the polymer compositions. It is indicated, however, that such additives can be initially incorporated in an inactive form, such as encapsulated or complexed with another compound, in order to avoid this undesirable aspect of their use. Ferric chloride, copper acetylacetonate, cuprous chloride, cupric nitrate, cupric acetate, copper acrylate, copper naphthenate, copper stearate, and antimony sulfate thus can be encapsulated with gelatin or other polymeric materials. It is also taught that these compounds can be complexed with nitro compounds or aromatic ethers or amines. A cited example of a suitable complex is an amine salt of ferric chloride. In some instances, certain iron, copper or antimony compounds normally detrimental to the stability or cure of the resin can be employed in small amounts.
An article in Plastics Engineering, February 1979, pages 43-47, refers to ferric chloride as a smoke suppressant for vinyl polymers. This article fails to note that ferric chloride operates to degrade the polymer during processing, and, of course, no method for curing this problem is suggested.
The foregoing prior art references show that iron and other metal compounds have been used as smoke suppressant additives for plastics. The most effective of these has been generally recognized to be iron oxide. Metal halides, while recognized as having some capacity as smoke suppressants, have nevertheless often been avoided since they cause premature degradation of halogentated polymers.